One of our clients is getting this error in the new version. She had no problem in the old version. She is using her home computer and does not have a firewall"
"The host from which you are accessing the board is not recognized as a valid host. This is more than liely related to a firewall issue blocking referer, check firewall."

What can we do so she can post to the BBS?

You can disable the referrer check in the control panel, which I would higly advise against as it'll allow any script or url to post (including spammers using malformed clients).

This issue at hand can come from more than just firewalls...
1. Firewalls
2. Anti-Virus Programs (and Suites)
3. Internet Security tools

A couple I've heard of the problem with:
1. Zone Alarm, both free and pro.
2. Norton (aka Symantec) Internet Security
3. Norton (aka Symantec) AntiVirus

There should be an option in the appropriate program to disable it from disabling (or not forwarding) the refferral address.